Sci-fi author John Scalzi published a blog post asking his fans to follow a very important piece of etiquette: When you attend a reading for his new book at a bookstore, you should buy the book at the bookstore the reading is taking place in.
The reason for this is simple: I want the store to see some benefit to me showing up. If you show up to the store with copies of the book that you bought elsewhere, and don’t otherwise buy any books from the store itself, then you’re not making a good argument for the store to bring in authors. And that will be no good for me, or for other authors who might otherwise come through town.
If you think it would kill you to buy the book at a price that isn't the absolute lowest available price, you should consider the author appearance to be part of the price of the book. That's a service that an online seller can't provide.